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  <title>Concept Covering and Archetypes</title>
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  <h2><a name="concept-covering" id="concept-covering">Concept Covering and
  Archetypes</a></h2>

  <p>We have discussed how it is important to select the minimal requirements
  (concepts) for the inputs to a component, but it is equally important to
  verify that the chosen concepts <i>cover</i> the algorithm. That is, any
  possible user error should be caught by the concept checks and not let slip
  through. Concept coverage can be verified through the use of <i>archetype
  classes</i>. An archetype class is an exact implementation of the interface
  associated with a particular concept. The run-time behavior of the
  archetype class is not important, the functions can be left empty. A simple
  test program can then be compiled with the archetype classes as the inputs
  to the component. If the program compiles then one can be sure that the
  concepts cover the component. The following code shows the archetype class
  for the <a href="http://www.boost.org/sgi/stl/InputIterator.html">Input
  Iterator</a> concept. Some care must be taken to ensure that the archetype
  is an exact match to the concept. For example, the concept states that the
  return type of <tt>operator*()</tt> must be convertible to the value type.
  It does not state the more stringent requirement that the return type be
  <tt>T&amp;</tt> or <tt>const T&amp;</tt>. That means it would be a mistake
  to use <tt>T&amp;</tt> or <tt>const T&amp;</tt> for the return type of the
  archetype class. The correct approach is to create an artificial return
  type that is convertible to <tt>T</tt>, as we have done here with
  <tt>reference</tt>. The validity of the archetype class test is completely
  dependent on it being an exact match with the concept, which must be
  verified by careful (manual) inspection.</p>
  <pre>
template &lt;class T&gt;
class input_iterator_archetype
{
private:
  typedef input_iterator_archetype self;
public:
  typedef std::input_iterator_tag iterator_category;
  typedef T value_type;
  struct reference {
    operator const value_type&amp;() const { return static_object&lt;T&gt;::get(); }
  };
  typedef const T* pointer;
  typedef std::ptrdiff_t difference_type;
  self&amp; operator=(const self&amp;) { return *this;  }
  bool operator==(const self&amp;) const { return true; }
  bool operator!=(const self&amp;) const { return true; }
  reference operator*() const { return reference(); }
  self&amp; operator++() { return *this; }
  self operator++(int) { return *this; }
};
</pre>

  <p>Generic algorithms are often tested by being instantiated with a number
  of common input types. For example, one might apply
  <tt>std::stable_sort()</tt> with basic pointer types as the iterators.
  Though appropriate for testing the run-time behavior of the algorithm, this
  is not helpful for ensuring concept coverage because C++ types never match
  particular concepts exactly. Instead, they often provide more than the
  minimal functionality required by any one concept. Even though the function
  template has concept checks, and compiles with a given type, the checks may
  still fall short of covering all the functionality that is actually used.
  This is why it is important to compile with archetype classes in addition
  to testing with common input types.</p>

  <p>The following is an excerpt from <a href=
  "./stl_concept_covering.cpp"><tt>stl_concept_covering.cpp</tt></a> that
  shows how archetypes can be used to check the requirement documentation for
  <a href=
  "http://www.boost.org/sgi/stl/stable_sort.html"><tt>std::stable_sort()</tt></a>.
  In this case, it looks like the <a href=
  "../utility/CopyConstructible.html">CopyConstructible</a> and <a href=
  "../utility/Assignable.html">Assignable</a> requirements were forgotten in
  the SGI STL documentation (try removing those archetypes). The Boost
  archetype classes have been designed so that they can be layered. In this
  example the value type of the iterator is composed out of three archetypes.
  In the <a href="reference.htm#basic-archetype">archetype class
  reference</a>, template parameters named <tt>Base</tt> indicate where the
  layered archetype paradigm can be used.</p>
  <pre>
{
  typedef less_than_comparable_archetype&lt; 
      sgi_assignable_archetype&lt;&gt; &gt; ValueType;
  random_access_iterator_archetype&lt;ValueType&gt; ri;
  std::stable_sort(ri, ri);
}
</pre>

  <p><a href="./prog_with_concepts.htm">Next: Programming with
  Concepts</a><br />
  <a href="./creating_concepts.htm">Prev: Creating Concept Checking
  Classes</a><br />
  <hr />

  <table>
    <tr valign="top">
      <td nowrap="nowrap">Copyright &copy; 2000</td>

      <td><a href="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>(<a href=
      "mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>) Andrew
      Lumsdaine(<a href="mailto:lums@osl.iu.edu">lums@osl.iu.edu</a>),
        2007 <a href="mailto:dave@boost-consulting.com">David Abrahams</a>.
    </tr>
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